2014-11-25
Mole
Chemistry course
ACME Faculty, EHVE course
B.Sc. Studies, I year, I semester
• Mole is a unit of matter quantity; it is used in similar context
as ‘milion’ or ‘thousand’ words.
• Milion of molecules is 1·106 molecules;
• Mole of molecules (atoms, particles) is 6.022·1023
• 6.022·1023 is called the Avogadro number (NA);
• Atomic and molecule mass (in a molecular scale) is expressed in ‘u’
(units) or atomic mass units.
Hydrogen atom weight is 1 u, water molecule weight is 18 u;
• 1 atom or molecule mole mass value in grams is equal to the given
atom/molecule in ‘u’;
• Hydrogen atom mole weighs 1 g (although hydrogen molecules – H2 2g). Water molecule mole weighs 18 g .
Molecular mass of the molecule is the sum of atomic masses
of which it consists.
Leszek Niedzicki, PhD, Eng.
Fundamental laws of chemistry
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Avogadro’s law
Reactions
• In different gases in the same volume
(at the same temperature/pressure) there is
the same number of gas particles (molecules
or atoms in case of noble gases).
• 1 mole of every gas occupies 22.4 dm3
(in normal conditions: T = 0˚C, p = 1013 hPa)
• ‘mole of a gas’ means ‘gas atoms’ in case of argon
(Ar) BUT ‘gas molecules’ in case of nitrogen (N2),
so 1 mole will be weighing 28 g, not 14 g.
• In real gases there are deviations from this rule,
but they are negligible (up to few percent at most).
• Reaction is process of new bond forming between
atoms (optionally after breaking previously existing bonds).
• Substances which existed before the reaction
and take part in it are called substrates.
• Substances which are the result of reaction are
called products.
A+B→C
H→I+J
• E.g.:
D+E→F+G
K+K→L
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Stoichiometry
Stoichiometry
• Quantitive ratios measured
(weighed) in human scale
(macroscale) are the same
as in the microworld (ratios between
• Molar ratio (mass ratio as well) of elements
in a compound is not always (especially in organic
chemistry) identifying unambiguously the compound.
E.g.: H2C=C2H (2C:4H = 1:2) i H2C=CH-CH3 (3C:6H = 1:2)
• Mass ratio (and molar
calculated from it) between
elements of a compound
can be used, although only
as one of few methods,
to confirm compound’s
structure (elemental
analysis).
atoms, molecules).
That means for the AxBy compound:
(molecules)
x·mA/y·mB = %A/%B
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(macroscale)
• Law of conservation of mass: mass
of substrates is equal to products
mass (mass of reagents is not changing).
• Mass ratio of elements
in a chemical compound is constant
and characteristic for a compound.
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Stoichiometry of chemical equations
Stoichiometry of chemical equations
• As a result of Avogadro’s law, volume ratio
of gases is the same as their molar ratio.
• Values of molar ratio that describe stoichiometry
of reagents are equal to stoichiometric
coefficients of the chemical equation.
• Quantity of one reagent is determining quantity
of all other reagents. That applies only to
substrates that reacted, not all that have been
put to the reaction
mixture (e.g. excess)
as well as products
resulting from reaction
• Substances are reacting with each other
in a certain ratio only, which is
characteristic for the given
reaction. Product is yielded
in a certain proportion
to substrates and to other
products, e.g.:
• 2H2+O2 → 2H2O
3H2+N2→ 2NH3
2 : 1 : 2
3: 1 :
2 (molar ratio)
4g: 32g : 36g
6g : 28g : 34g
(4+32=36
6+28=34)
(including any possible side
products).
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Energy of reaction
Stoichiometry of chemical equations
• In order for reaction to start, it has to be "beneficial“
for substrates.
After reaction they have to be in "better" (energetically)
state than before it. Reagents' surroundings (conditions)
have to be beneficial towards reaction initiation as well
Example:
How many moles of oxygen is needed to fully
combust 6 moles of hydrogen?
Basic reaction equation:
2H2+O2 → 2H2O
So to combust 6 moles
of hydrogen:
6H2+3O2 → 6H2O
Answer: Three moles of oxygen are needed
(ca. 67.2 dm3)!
(proper conditions mean: temperature over certain level, pressure,
presence of certain catalyst or absence of inhibitor). Often it is
critical for reaction to start (required, not beneficial).
Reaching such conditions often requires a lot of energy.
• Some reactions emit energy (heat), some absorb energy
(heat). The latter decrease the temperature of its
surroundings if not heated artificially (from outside).
• The heat (energetic) effect is characteristic for the given
reaction (at the constant pressure). This effect is called
the enthalpy of reaction (ΔH).
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Energy of reaction
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Mixture and chemical compound
• ΔH > 0 means that reaction is endothermic
(absorbs energy).
• ΔH < 0 means that reaction is exothermic
(emits energy).
• Minimal requirements of reaction initiation
mentioned earlier can be of many reasons.
Among others (boundary reasons): above certain
temperature one of substrates can be changing
its state of matter or decomposing (explodes);
below certain temperature solvent, in which
reaction takes place, can be solidifying or one
of substrates can be precipitating, etc.
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• Chemical compound has defined
composition and all its atoms are
connected with chemical bonds.
• Mixture is two or more
compounds/elements mixed with
each other. They can be of the same
state of matter or not. Mixture
composition can be changed, e.g.
by adding some of one of the
components. Atoms of different
compounds are not bonded with
each other, but can be (do not have to)
interacting with each other.
Wikimedia Commons
CC BY-SA 3.0 Materialscientist
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Mixtures
• Mixture component can be of any state of matter.
• Mixtures can be homogenous or not - heterogenous.
• Phase is a physically uniform part of the system
separated from other phases with interfacial boundary
(interphase), at which there is nonlinear, abrupt
change of properties (physical, chemical or both).
– Oil poured on the water surface forms a mixture
with a clearly visible phase boundary (both phases are
liquid).
– Mist (atmospheric phenomenon) is the mixture
of microscopic water droplets (liquid phase) surrounded
with water vapor (gaseous phase).
– Water containing dissolved sugar is a two-component
(neglecting impurities), but one-phase mixture
(homogenous).
Mixtures
• Mixture consist of dispersing phase
and of dispersed phase.
• Mixtures can be divided
into the following categories:
– heterogeneous (dispersed phase is visible with a naked eye
or a magnifying glass)
– coloid (dispersed particles are of 1nm-500nm size).
– homogenous = solutions (dispersed particles are below
1nm size)
• In homogenous phases dispersing phase is called
a solvent, the other components are called solutes.
• Solvent has to be of the same state of matter as the final
mixture (solution).
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Colloids
Heterogeneous mixtures
Components of mixtures retain their
both macro- and microscopic
properties; they can be separated with
simple physical methods, e.g.:
filtration, segregation, centrifugation
(cyclone), etc.
Wikimedia Commons CC BY 3.0 H. Żychowski
Wikimedia Commons CC BY 3.0 Σ64
Dispersed phase
gas
Wikimedia Commons CC BY-SA 3.0 I. Sagdejev
liquid
solid
Solvent
Dispersed phase
Solvent
gas
liquid
solid
gas
-
spray (drizzle)
smoke (combustion
gases with ash)
liquid
foam (soap foam)
emulsion
(water with oil)
suspension (water
with sand = mud)
foam (styrofoam)
emulsion
(composite
electrolytes)
- (some alloys,
composites,
minerals, etc.)
solid
• Mixtures that are something
in between homogenous
and heterogenous ones.
• We can observe Tyndall's
effect in colloids.
gas
-
liquid aerosol (mist)
solid aerosol
(smoke)
liquid
liquid foam
(whipped cream)
emulsion (milk)
liquid sol
(water with chalk)
solid
solid foam (some
composites)
gel (some polymers
with plasticizers)
solid sol (some
alloys and
composites)
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Solutions
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Solutions
• Components in solutions lose some of their
individual properties. Solution is physically
homogenous. Sample taken from any point of the
solution will have the same composition
and properties as the whole solution
(components ratio is the same for the whole
volume of a solution).
• Components of solution can
be separated, e.g. by means
of destillation, crystallization
or chromatography.
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• Dissolving is a process of mixing particles of one
component (dispersed phase = solute) among
particles of other component (solvent).
• Dissolving rate can be controlled through:
temperature change (higher = faster dissolution),
stirring (faster/more intensive = faster dissolution)
and comminution (higher interfacial surface = faster
dissolution).
• Solution composition can be changed by solute
addition, solvent addition (dilution) or solvent
evaporation (expelling/concentrating).
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Solubility
• Is a maximal concentration of a given substance
in a given solvent (at given temperature and in case
of gases, at given pressure).
• Solution at the peak concentration is saturated.
Solute addition to saturated solution does not
change its concentration. Equilibrium
is obtained instead, at which
dissolution is taking place
at the same rate as
crystallization of solute..
• Solubility is measured in
grams of solute per 100 g
of solvent (depends
on temperature).
Solubility
• Unsaturated solutions are solutions
with concentration lower then the peak one
(at given temperature).
• Supersaturated solutions are instable
(metastable) solutions with a concentration
higher than the peak one (obtained by e.g. very slow
evaporation of the solvent from unsaturated solution).
Upon nucleation initiation (e.g. through small solute
crystal addition, etc.) very quick crystallization of the
solute excess takes place.
• Solubility of liquids and solids usually increases
with a temperature. Solubility of gases decreases
with a temperature.
www.flickr.com/photos/cmbellman/2561965334
CC BY-NC-ND 2.0 A. Adermark
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Concentration
Solubility
• Percentage concentration (written as % or weight%):
• Dissolution influences:
=
· 100%
+
• Molar concentration (note that denominator consists
of volume of whole, final solution!):
– density change (mass increase with small volume change);
– contraction (sum of components’ volume is not equal to mixture
volume - Mendeleev!);
– melting point value decrease (pouring salt on ice-covered
roads/sidewalks during winter) – eutectics;
– boiling point value increase;
– thermal effect (depends on mixing enthalpy) – „always pour
=
unit: mol/dm3 or „M”
• Molal concentration (used when, for instance, solvent
is a mixture of solvents):
acid into water, never the other way around”;
– dissociation/solvation.
=
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unit: mol/kg or „m”
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Concentration
• What is the percentage concentration of 10 g
kitchen salt (NaCl) solution in two liters
of water?
• What is the molar concentration of 11,7 g kitchen
salt (NaCl) solution in a liter of water (MNaCl = 53,5
g/mol)?
• What is the molality of 6,84 g sucrose (which
molecular mass is 342 g/mol) solution in 250 cm3
of water?
• What is the percent concentration of copper and
zinc in an alloy that consists of 30 g of copper
and 120 g of zinc?
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